Gre'thor

Gre'thor is the name of the underworld, or Hell, in Klingon society. It is the location where the dishonored and the damned would journey to, once they died.

Once the dishonored died, the Klingon would first board the Barge of the Dead which was commanded by Kortar, the first Klingon who was banished to this realm for his act of destroying the Klingon gods. The Barge would then set sail on the blood red river for Gre'thor itself but this would not be a peaceful journey as dangers, such as the serpentine Kos'karii would attempt to lure the warrior into the water with their siren-like voices in order to get them lost for eternity. (VOYepisode: "Barge of the Dead")